The casting of articles from molten metal in cavities formed in sand molds is a commonly used procedure. Generally, this procedure involves forming a mold cavity by packing sand around a pattern, removing the pattern, pouring molten metal into the cavity and then allowing the metal to cool and solidify. Each mold is used only once.
In cases of a high production demand for specific cast articles, a common procedure has been to make the molds of permanent materials, such as steel or iron or graphite, within which the casting cavities have been permanently machined. Here, each mold may be reused a number of times. The use of permanent molds depends, to a considerable extent, upon the quantity of production, the nature and size of the cast article to be produced, the expenses for making the molds, etc.
In the casting of ferrous metals, attempts have been made in the past to combine the benefits of sand casting molds, with the longevity of and ease of handling of permanent molds, by applying a sand or sand-like coating over the cavity wall within a permanent mold. In high production casting in permanent molds, it is desirable to cool the molten metal as rapidly as feasible so as to more rapidly remove the finished casting from the mold. But, since sand is an insulating material, it slows the cooling of the cast article within the permanent mold as compared to the more rapid cooling in an uncoated permanent mold.
Moreover, where the cast article has sections that are thicker than other sections, the thicker sections normally cool more slowly than the thinner sections. Thus, the thicker sections determine the length of time needed before the finished article can be removed from the mold. Because the thicker sections cool more slowly than the thinner sections, structural and metallurgical problems have been encountered due to the differential in cooling between integral thick and thin sections resulting in part of a casting being substantially cooled and adjacent parts being less cooled at the same time. The use of sand in permanent mold cavities tend to increase the differential cooling problems. Thus, the invention herein is concerned with the utilization of permanent molds whose cavities are sand lined, but wherein the thicknesses of the various parts of the sand liner are varied in a way that causes varying cooling of the cast article within the mold so as to permit a faster casting and removal cycle and to produce a better cast product.